1. Field of the Invention
The present invention relates to apparatus for the measurement of optical path length changes between two plane mirror surfaces. More particularly, the invention relates to optical apparatus which is useful for high accuracy displacement metrology using interferometry.
2. The Prior Art
An interferometer is a basic instrument for most high accuracy displacement measurements in dilatometry, material stability studies, the machine tool industry, and in the semiconductor fabrication industry. One type of interferometer representative of the current state-of-the art is the differential plane mirror interferometer which measures the optical path length changes between two external mirrors and which is described in R. R. Baldwin and G. J. Siddall, "A double pass attachment for the linear and plane interferometer," Proc. SPIE, Vol. 480, pp. 78-83 (May 1984). A conventional differential plane mirror interferometer consists of a fixed plane mirror and a movable plane mirror, which form the interferometer cavity, and auxiliary optical components (retroreflectors, wave plates, mirrors, beamsplitters). This type of interferometer has an inherent optical resolution of one quarter of the wavelength of the light used and has particularly high stability which is necessary for the ever increasing demand for improved accuracy. Thusly it is particularly insensitive to any tilt of the plane mirrors and motion of the auxiliary optic components.
References, M. Okaji and H. Imai, "High-Resolution Multifold Path Interferometers for Dilatometric Measurements," J. Phys. E: Scientific Instruments, Volume 16, pp. 1208-1213, 1983, and M. Okaji and H. Imai, "A Practical Measurement System for the Accurate Determination of Linear Thermal Expansion Coefficients," J. Phys., E: Scientific Instruments, Vol. 17, pp 669-673, 1984, illustrate other embodiments of differential plane mirror interferometers.
The conventional differential plane mirror interferometer is, however, overly complicated, requiring many auxiliary optical components thereby subjecting the measurement beams to many reflections. These drawbacks ultimately limit the accuracy that can be achieved due to a lower signal-to-noise in the measurement signal as a result of reduced optical beam power and polarization leakage.
In commonly owned, copending U.S. patent application Ser. No. 810,999, entitled "Differential Plane Mirror Interferometer", filed Dec. 19, 1985, the contents of which are specifically incorporated by reference herein, an improved differential plane mirror interferometer is disclosed in which the use of a shear plate not only reduces the number of optical elements but also reduces the number of reflections by nearly 50%.
The present invention retains the basic plane mirror interferometer cavity of the conventional differential plane mirror and the simplicity of the copending shear plate interferometer described in the aforementioned copending U.S. patent application Ser. No. 810,999; however, it uses a beamsplitter/beam folder assembly to generate and recombine two separated, parallel, orthogonally polarized beams instead of the previously disclosed shear plate. Since the optical efficiency of the prism elements of the present invention can be higher than that of the previously disclosed shear plate, the signal-to-noise and, thusly, the number of interferometers which can be used with a single laser source can be greater with this type of system.